I replied to a similar post a short while back. The G50's seem to be in high demand. I had one in my car and when changing it to a G64/51 the G50 had a taker without me having to even suggest that it might be for sale.

If you cannot find an individual who can sell you one, it looks to me that van Zweeden in Holland has a lot of G50's available. Patrick Motorsports would definitely be able to help you here in the US. I don't know if they deal with used trannies though whereas van Zweeden does. If you end up getting it from overseas, the shipping will probably be close to a $1,000.
That's what my shipping was from Dubai (although thanks to the good will of the seller, I myself only paid less than half of it).